Recently, a global trend toward stricter quality regulation values for gas oil has emerged to improve atmospheric environments. In particular, a sulfur reduction in gas oil is desirable because sulfur may adversely affect the durability of after-treatment devices expected to provide effective control of diesel emissions, such as oxidation catalysts, nitrogen oxide (NOx) reduction catalysts, and continuous-regeneration type filters for removing particulate matter from diesel exhausts.
Under these circumstances, it has been emphasized to develop an ultra-deep desulfurization technology for removing most of the sulfur components in gas oil substantially. A generally possible technology for reducing the sulfur components in gas oil is to use severer operating conditions for hydrodesulfurization, e.g., reaction temperature and liquid hourly space velocity.
However, use of an elevated reaction temperature results in precipitation of a carbonaceous matter on the catalyst and hence in a rapid decrease in catalytic activity. On the other hand, use of a lowered liquid hourly space velocity results in a reduced purification capacity although desulfurization ability is improved, so that it becomes necessary to enlarge the scale of the facility.
Consequently, the best way of attaining the ultra-deep desulfurization of gas oil without using severer operating conditions is to develop a catalyst having an excellent desulfurization activity.
Recently, many investigations have been made on kinds of active metals, methods of active-metal impregnation, improvements of catalyst supports, regulation of catalyst pore structures, activation methods, and the like, and novel catalysts for deep desulfurization developed have been reported.
For example, Patent Document 1 discloses a process which comprises impregnating an alumina or silica support with a solution which contains an organic compound having a nitrogen-containing ligand as a complexing agent and further contains an active metal, followed by drying at 200° C. or lower.
Patent Document 2 discloses a process which comprises impregnating a γ-alumina support with an impregnating solution obtained by further adding a diol or an ether to an impregnating solution containing a compound of a metal in the Group 8 of the periodic table (hereinafter, sometimes simply referred to as “Group 8 metal”), a compound of a metal in the Group 6 of the periodic table (hereinafter, sometimes simply referred to as “Group 6 metal”), and phosphoric acid, followed by drying at 200° C. or lower.
Patent Document 3 discloses a process which comprises impregnating a support with a solution comprising a compound of a Group 6 metal, a phosphorus component, a compound of a Group 8 metal and citric acid, as in the process of the present invention, followed by burning without drying. Patent Document 4 discloses a process which comprises impregnating a support, on which a compound of a Group 6 metal, a phosphorus component and a compound of a Group 8 metal have been supported, with a solution containing a specific amount of an organic acid, followed by drying at 200° C. or lower.
Furthermore, Patent Document 5 discloses a process which comprises supporting a solution containing a compound of a Group 6 metal, a compound of a Group 8 metal, and phosphoric acid on an oxide support, drying the resulting support at 200° C. or lower to obtain a catalyst, supporting a solution of an organic acid represented by a specific chemical formula on the catalyst, and then drying at 200° C. or lower.
On the other hand, proposals have been made also on a process for catalyst production wherein impregnation is conducted twice with an organic acid.
For example, Patent Document 6 discloses a process which comprises impregnating an oxide support with a solution comprising a compound of a Group 6 metal, a compound of a Group 8 metal, organic acid and phosphoric acid, followed by drying at 200° C. or lower to obtain a catalyst, and further impregnating the catalyst with a solution of an organic acid, followed by drying at 200° C. or lower.
In addition, Patent Document 7 discloses a technique for catalyst production which comprises impregnating an inorganic oxide support with a compound of a Group 8 metal and heteropoly acid of a Group 6 metal, followed by drying.
Moreover, Patent Document 8 discloses a process for catalyst production which comprises impregnating an oxide support with a solution comprising molybdenum, tungsten, a compound of a Group 8 metal, mercaptocarboxylic acid, and phosphoric acid.
This process is mainly intended to form a coordination compound of the mercaptocarboxylic acid with molybdenum, tungsten, and the Group 8 metal compound to highly disperse the coordination compound on the catalyst support.
However, in the process described above, the molybdenum and tungsten is highly dispersed on the support and, hence, it is difficult to form laminated layers of molybdenum disulfide such as catalysts in the present invention which will be described later. It is presumed that the process results in no formation of Type II sites of a CoMoS phase, NiMoS phase, or the like which are especially effective as active sites for desulfurization (i.e., active sites of cobalt or nickel located at the edges of the second and overlying layers of molybdenum disulfide; Type I sites mean the active sites of cobalt or nickel located at the edges of the first layer of molybdenum disulfide, and have activity lower than the Type II sites).
In addition, since the mercaptocarboxylic acid contains sulfur, there is a possibility that when the acid is present around the Group 8 metal (Co or Ni) or forms a coordination compound, then the acid may give not desulfurization-active sites (CoMoS phase, NiMoS phase, or the like) but inactive Co9S8 or Ni3S2 species.
Also, the processes for catalyst production described above have drawbacks that some of these necessitate complicated steps and that some of the catalysts obtained are unsuitable for use in the ultra-deep desulfurization of gas oil, some exhibit a low efficiency in the ultra-deep desulfurization range, and some have a short life.
Moreover, Patent Document 9 discloses a catalyst comprising a salt and/or complex of a Group 8 metal selected from cobalt and nickel and a heteropoly acid of a Group 6 metal selected from molybdenum and tungsten on an oxide support, wherein the concentration of the Group 8 metal is about from 2 to 20% by weight based on the support, the concentration of the Group 6 metal is about from 5 to 50% by weight based on the support, and the catalyst substantially contains no free water. Furthermore, Patent Document 10 discloses a catalyst obtained by adding a hydroxycarboxylic acid, in an amount of 0.3 to 5 equivalents to total moles of the metals of a Group 6 metal and a Group 8 metal, to a catalyst in which the Group 6 metal and the Group 8 metal are supported on a support, and subsequently drying at a temperature of 200° C. or lower.
Although various catalysts as results of development have been reported as mentioned above, there is still no proposal of a technology for obtaining a catalyst having a sufficiently high desulfurization activity and a long life, which is capable of being produced by a simple method and also capable of realizing the ultra-deep desulfurization of gas oil without using severer operating conditions.
[Patent Document 1]
JP-A-61-114737
[Patent Document 2]
Japanese Patent No. 2900771
[Patent Document 3]
Japanese Patent No. 2832033
[Patent Document 4]
JP-A-4-156948
[Patent Document 5]
JP-A-4-244238
[Patent Document 6]
JP-A-6-339635
[Patent Document 7]
JP-A-6-31176
[Patent Document 8]
JP-A-1-228552
[Patent Document 9]
JP-A-6-31176
[Patent Document 10]
Japanese Patent No. 3244692